I have an English question about grammar.

I am in a cafe preparing an English presentation for next Wednesday.

My question point is "that" located between "cafe" and "is".
I think the word "that" should be located right after "presentation" which is related to "is going to be conducted in English next Wednesday". However, I want to put "at a cafe" because I'm actually preparing the presentation at a cafe, but in my sentence, the last part of a sentence seems to modify the word "cafe" because it is located right after "cafe".

It's hard for me to explain this in English. To make things easier, we may normally say like "The desk that I bought yesterday was very expensive."
Then, can I also say "The desk was very expensive that I bought yesterday." ?

I'm curious if the sentence below is correct.

Very often, we use relative clauses (clauses that start with a relative pronoun such as "that") to provide more information about a noun. In terms of sentence construction, the relative clause often comes immediately after the noun that we would like to describe further.

In your sentence, your emphasis is on providing more information on "a presentation", and according to you, "at a cafe" is a prepositional phrase. **Therefore, you may write: "I'm in a cafe preparing for a presentation (noun which you would like to provide more information on) that (relative pronoun) is going to be conducted in English next Wednesday." Notice that the relative pronoun is introduced right after the noun that you are elaborating on. 

It's hard for me to explain this in English. To make things easier, we may normally say like "The desk that I bought yesterday was very expensive."

Then, can I also say "The desk was very expensive that I bought yesterday." ?

No, you cannot do so as the sentence structure is wrong. Using the structure which I have previously mentioned, the sentence should be written as: "The desk (noun you would like to elaborate on) that (relative pronoun) I bought yesterday was very expensive.

Another way to explain is that relative clauses are actually adjectival clauses which act like adjectives describing a noun. Since "very expensive" and "I bought yesterday" are both describing the noun, they should come after "that": The desk that (relative pronoun) I bought yesterday (describing when the desk was purchased) was very expensive (describing the cost of the desk).

Hope this helps.

Cheers, 
Lance

Very well done for noticing this! Ten out of ten! Many people (native speakers and English learners alike) often do not seem to notice that they sometimes use relative pronouns incorrectly, and as a result they often end up producing sentences that are nonsensical. So, well done to have spotted the problem and to have asked about how to sort it!

Some great answers already.

I would say that:

"I'm preparing a presentation at a cafe that is going to be conducted in English next Wednesday"

should be changed to:

"I'm at a cafe preparing a presentation that is going to be conducted in English next Wednesday"

and that will solve your problem.

No, in this case, the change you have made results in a nonsensical sentence. Relative pronouns (words like that, which, who, and whom) are used after nouns. They are not used after adjectives. "Expensive" is an adjective, not a noun. As a result, your proposed sentence sounds very odd, and is not correct at all.

Using a relative pronoun after "desk" is fine, because "desk" is a noun. However, you cannot use a relative pronoun after "expensive" because "expensive" is an adjective.

Hope this helps. :)